Failure of phrenic nerve-diaphragm neuromuscular transmission leads to hypercapnic respiratory failure. This occurs not only in overtly diseased neuromuscular junctions (eg. myasthenia gravis, botulism), but also in normal junctions subjected to high intensity activation during mechanical loading by lung disease (eg. COPD) or during exposure to systemic factors such as hypoxia and hypothermia. Neurotransmission requires sufficient prejunctional release of acetylcholine (ACh) to ensure muscle contraction. During repetitive activation, ACh release diminishes, which when severe leads to transmission failure. Restoration of ACh available for release depends on two separate but interrelated processes: recycling of transmitter from the synaptic cleft, and repletion of the immediately releaseable vesicle pool from one or more reserve pools. Respiratory muscles are active continuously, so that transmitter replenishment needs to be sufficiently robust to ensure that a constant supply of ACh is available for release. The overall objective of this proposal is to further examine the role of transmitter replenishment, and the factors which regulate replenishment, in determining the integrity of transmission in respiratory neuromuscular junctions. The specific hypotheses to be tested are as follows. 1) The rapidity of, and time available for, ACh replenishment are critical determinants of transmission at the phrenic-diaphragm neuromuscular junction, especially in diseased neuromuscular junctions. 2) ACh replenishment is hastened by high frequency stimulation, an accommodation to the adverse effects of high frequency activation on release and depletion. Furthermore, the acceleration of replenishment is mediated by elevated presynaptic [Ca+ +]. 3) Hypoxia and hypothermia impair neurotransmission to a large extent by slowing transmitter replenishment, rather than primarily by a direct inhibition of transmitter release. 4) Presynaptic K+ channels regulate not only Ach release but also transmitter replenishment, providing two mechanisms of improving neurotransmission by pharmacologic manipulation of K- channel conductances. Neuromuscular transmission will be assessed using a combination of force measurements to quantify the neuromuscular component of fatigue, electrophysiological recording to determine ACh release and recovery from transmitter rundown, and optical approaches using fluorescent styry1 dyes ( FM1-43, FM2-10) to assess vesicle pool dynamics. These studies may lead to novel therapeutic approaches to respiratory muscle impairment and resulting hypercapnic respiratory failure for conditions which produce neuromuscular junction dysfunction.